This invention relates to a fluidized bed reduction method used in the reduction of powder raw materials including powder ore or partially pre-reduced powder ore, and a fluidized bed reduction reactor and fluidized bed reduction system which can be used in the fluidized bed reduction method.
Fluidized bed reduction methods have been used conventionally as methods for reducing powder raw materials including powder ore or partially pre-reduced powder ore, and there exists many prior patents and publications disclosing the same, such as U.S. Pat. Nos. 5,118,479, 5,382,277, 5,431,711, 5,529,291 Japanese Patent No. 2536339, Japanese Patent No. 2536641, and a pamphlet published in 1987 by Fior de Venezuela, S. A.
Amongst the systems used in these fluidized bed reduction methods, several systems have been proposed with respect to reactors inside of which the powder raw material is made to form a fluidized bed and reduced. The first fluidized bed reduction methods involved securely maintaining a fluidized bed formed inside a single chamber, and carrying out reduction in this position. However, in recent years, the focus has been on systems in which the aim is to sequentially move the fluidized bed in line with the increase in degree of reduction, and carry out reduction efficiently in step with this movement. Such a system is disclosed in, for example, U.S. Pat. No. 5,118,479, in which, as shown in FIG. 1, there are arranged a plurality of guide plates for directing the movement of the fluidized bed in a single chamber in a zig-zag manner. In the pamphlet published by Fior de Venezuela, S. A. in 1987, there is disclosed a system in which a plurality of fluidized bed chambers are used, and in which the powder ore is reduced in each fluidized bed chamber whilst being moved between fluidized bed chambers.
With the reactor of the type shown in FIG. 1, powder ore A is introduced into fluidized bed chamber 1 from inlet 2, and forms a fluidized bed with the reducing gas introduced from reducing gas inlet 3 whilst moving in a zig-zag manner along guide plates 4, 5, 6, 7. The reduced powder B is removed from outlet 8. The temperature of the fluidized bed when effecting this system is usually set to be 700.degree. C. or more. At these kinds of temperatures, the guide plates 4, 5, 6, 7 deform through thermal expansion, making it difficult to stably maintain the guide plates in an upright state. If the guide plates 4, 5, 6, 7 are not maintained in an upright state, this can have a bad effect on the state of the moving fluidized bed. Furthermore, in order to increase the reducing power, it is necessary to enlarge the fluidized bed chamber 1 and increase the number of guide plates in order to make the distance of movement of the fluidized bed inside the reactor sufficiently long. In such a case, in addition to the problem of the deformation of the guide plates, there is the fear that as the reducing gas inlet 3 and support components are enlarged, the bad effects of thermal expansion will increase all the more causing other problems with respect to the pressure-resistance, airtightness etc.
FIG. 2 shows a generalized view of a reactor in which several fluidized bed chambers are employed, and the reduction reaction is carried out sequentially in each fluidized bed chamber, as the powder ore is moved between fluidized bed chambers. Powder ore A is introduced from inlet 9 into the first fluidized bed chamber 13a, and moves along connection passages (16a, 16b) whilst forming a fluidized bed in each of the fluidized bed chambers (13a, 13b, 13c). The reduced powder B is removed from the last fluidized bed chamber 13c via outlet 17. The reducing gas is first introduced into the last fluidized bed chamber 13c from reducing gas inlet 14. Thereafter, it is exhausted from above the fluidized bed in fluidized bed chamber 13c, and introduced via reducing gas line 15a into the fluidized bed chamber 13b which comes one before it in the direction of the movement of the fluidized bed. Similarly, thereafter, the reducing gas which has been used in the fluidized bed chamber 13b is introduced into the fluidized bed chamber 13a via reducing gas line 15b. The reducing gas which has been used in the first fluidized bed chamber is exhausted via gas exhaust line 10. With this kind of system, the above-described deformation problems tend not to occur. Furthermore, since it is possible to meet the desired degree of reduction by increasing or reducing the number of fluidized bed chambers, those problems with respect to pressure-resistance and airtightness which can be associated with enlargement of the fluidized bed chamber (13a, 13b, 13c) also tend not to occur. In addition, since the system involves a construction in which the fluidized bed chambers are connected in series with respect to the flow of the reducing gas, the efficiency of use of the reducing gas is high. However, since the reactor system is constructed such that powder ore which has overflowed from the surface of the fluidized bed in each fluidized bed chamber moves to the next fluidized bed chamber, there is the problem that whereas relatively fine powder is easily moved, relatively large powder particles tend to sink to the bottom of the fluidized bed with the result that they tend to become trapped in a single fluidized bed chamber. This would not be such a big problem if the relatively large trapped powder particles would become suitably reduced, and if they would degenerate into fine powder in the fluidized chamber during the time that they are trapped and then sequentially move through the series of fluidized bed chambers. However, there exists a large difference in the extent of reduction between trapped material and material which moves smoothly, with the result that the smoothness of the reduction reaction is lost, and in some cases, successively trapped large powder particles collect in the bottom of the fluidized bed, which has a bad effect on the movement of the fluidized bed and on the flow of the reducing gas. If this occurs, then the load on the particular fluidized bed chamber in which it occurs increases compared to other fluidized bed chambers. Furthermore, since the reducing gas is caused to flow in series, there is the fear that there will also be a bad effect on the reducing efficiency of the system as a whole. Furthermore, with this kind of system, the reduced powder material is removed without exception from the outlet 17 of the last fluidized bed chamber 13c. This is unavoidable in order to obtain material having a sufficiently high degree of reduction, due to the fact that the reducing power of the reducing gas used in the last fluidized bed chamber is the greatest. However, it may occur that the powder has attained the prescribed degree of reduction before reaching the last fluidized bed chamber 13c, but due to the fact that the construction does not allow the removal of the ore before the last fluidized chamber 13c, the powder has to be passed without exception to the last fluidized chamber 13c which is not very efficient in terms of time and operation.